President Barack Obama described the Boston Marathon bombings as attacks by "two brothers and a crock pot" on Monday as an example of the difficulty of stopping terrorist attacks.

Obama, speaking after a meeting with world leaders in Turkey, conceded that the Paris terror attacks were a "terrible and sickening setback" in the fight against the Islamic State.

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The president said that the attack on the Boston Marathon shows that even a small number of people can terrorize an entire region.

“That was an attempt at killing a lot of people by two brothers and a crock pot,” Obama said. “It gives you some sense of the kinds of challenges that can be involved in this going forward.”

The Tsarnaev brothers used pressure cooker bombs to kill and maim those at the finish line in 2013.

The president grew irritated amid repeated questions about whether he had underestimated the strength of the Islamic State.

He said most of his critics are simply "talking as if they're tough" and offering no real ideas. And he brushed aside those who call for sending U.S. ground troops into the region, saying that "would be a mistake" and wouldn't work unless the U.S. was committed to being a permanent occupying force in the region.

"This is not an abstraction," Obama said. "When we send troops in, those troops get injured. They get killed."

While Obama did not single out his critics by name, some Republican presidential candidates, including former Florida Gov. Jeb Bush, have called for sending U.S. forces into Syria. Bush has also suggested that any U.S. assistance to refugees fleeing the Middle East should be primarily focused on Christians, another idea that rankled Obama.

"That's shameful," Obama declared. "That's not American. That's not who we are."

Rather than casting about for a new strategy, Obama said U.S. would intensify its current campaign of airstrikes and arming and training moderate forces. And he called on other nations to step up their involvement in the fight against the extremists.